Superconductivity is that characteristic of certain materials which permits them to conduct electric currents without resistance. A superconducting material exhibits this characteristic only when its temperature is below the superconducting critical temperature of the material, and then only if it is not subject either to a magnetic field greater than the superconducting critical magnetic field of the material or to an electric current density greater than the superconducting critical current density of the material. Accordingly, superconductivity can be quenched, i.e., returned to a resistive state, by increasing the temperature, magnetic field, or current density to which the superconducting material is subjected above the critical temperature, magnetic field, or current density of the superconducting material.
The discovery of a new superconductor comprised of oxides of lanthanum, barium, and copper by J. G. Bednorz et al., "Possible High-T.sub.c Superconductivity in the Ba--La--Cu--O System," Z. Phys. V. 64, pg. 189, 1987, led to the discovery of a series of superconducting oxide compounds having a high critical temperature above 30 K. For example, a bismuth system of oxide compounds, containing oxides of one or two gram atoms of bismuth, one or two gram atoms of calcium, one or two gram atoms of strontium, and one, two or three gram atoms of copper, have a critical temperature above 7 K., and are herein referred to as bismuth superconductor oxide compounds.
The oxide superconductors, for example the oxide compound of bismuth, strontium, calcium, and copper in the ratio of about 2: 2: 2: 3, respectively, can be shown by a formula, Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10+y. The cation ratios given in such formulas usually indicate the ideal structure, and the exact oxygen content per molecular unit is nominal so that y varies within about a fraction of one. In addition, there may be substitution of cations on other sites, cation vacancies, or oxygen interstitials present so that the actual superconducting phase is not exactly that given by the ideal formula.
The discovery of the oxide superconductors enables the development of superconducting equipment operating at temperatures up to liquid nitrogen cooling, about 77 K., instead of liquid helium cooling, about 4.2 K. Among the applications for such oxide superconductors are power transmission lines, rotating machinery, and superconducting magnets, e.g. for fusion generators, particle accelerators, levitated vehicles, magnetic separation, energy storage, and magnetic resonance imaging. These devices require the development of elongated conductors, e.g., wire or tape of the oxide superconductors.
However, to be of practical value an oxide superconductor must be able to carry or conduct a substantial current, at least approaching the current-carrying capacity of alloy superconductors such as Nb.sub.3 Sn. The high current-carrying capacity must be evident at temperatures up to the critical temperature without quenching or loss of superconductivity as evidenced by a substantial increase in electrical resistance.
It is known that the oxide superconductors have an orthorhombic crystal form that exhibits anisotropy in the critical current density. The a- and b- directions within the basal plane of the orthorhombic crystal are capable of supporting on the order of 30 to 100 times the critical current density which can be achieved in the c- direction normal to the basal plane. As a result, one approach to improving the critical current density of a polycrystalline oxide superconductor is to form a single phase powder of the oxide superconductor, and align the powder through some mechanical working operation such as extrusion, wire drawing or rolling.
For example, U.S. Pat. No. 4,973,575, discloses a method of orienting the crystalline axes of different grains in oxide superconducting materials along the basal plane to facilitate current transport across the grain boundaries. A single crystal powder is formed by grinding into fine powders either bulk single crystals, or a powder formed by long term annealing of powder samples of the polycrystalline oxide material at temperatures near the melting temperature to promote large single crystal grain growth. In grinding the superconducting crystals to a size less than the original grain size, the compact superconducting metal oxide material is cleaved along the crystal axes resulting in platelets, having a disc-like shape. The axes of the platelets having a high critical current density lie in the plane of the disc-like shape. Conventional mechanical working techniques, such as extrusion, tape casting or slip casting are used to align the superconducting particles and increase the critical current density of the polycrystalline oxide superconductor body sintered therefrom.
In another method of forming aligned polycrystalline oxide superconductors, European patent application 357,779 discloses a metal tube is filled with a powder of an oxide superconductor having a perovskite structure. The tube is reduced, for example by rolling by more than 30 percent, to form a compressed oriented layer of the powder oriented so the c-axial direction is perpendicular to the longitudinal direction of the conductor. The reduced tube is heated to sinter the oxide superconductor powder.
One aspect of this invention is a method for forming an aligned polycrystalline Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8+y oxide superconductor core, within a normal metal sheath.
Another aspect of this invention is a composite elongated conductor having a normal metal sheath, and an oxide superconductor core having an aligned polycrystalline core of Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8+y.
In the oxide superconductor having the approximate formula Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8+y, it is known that lead can be partially substituted for bismuth, and the lanthanides can be partially substituted for strontium or calcium. As used herein, the term "lanthanide" means the elements having an atomic number from 57 to 71 in the periodic table, and yttrium. Therefore, the Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8+ y oxide superconductor can be shown by the approximate formula Bi.sub.2-x Pb.sub.x Sr.sub.2-a L.sub.a+b Ca.sub.1-b Cu.sub.2 O.sub.y where y is from 7.5 to 8.5, and L is a lanthanide, and is sometimes herein referred to as the "2212 compound".